This invention relates to an improved system for controlling the recirculation of exhaust gas through an internal combustion engine.
Concerning the prevention of air pollution by exhaust gas of internal combustion engines, probably a most widely employed technique for suppressing NOx emission is the recirculation of a portion of exhaust gas back into the engine intake. The recirculation of exhaust gas (EGR) has the effect of lowering the maximum combustion temperature in the engine combustion chambers so that the formation of NOx in the combustion chambers can be suppressed.
The suppressive effect of EGR on the formation of NOx is enhanced as the volume of the recirculated exhaust gas relative to the volume of fresh air admitted into the engine is increased (this volume ratio will hereinafter be referred to as EGR rate). To maintain NOx emission below a permissible level, there is a need of effecting EGR at considerably high EGR rates. On the other hand, the employment of high EGR rates tend to cause instability of the engine operation. Accordingly the EGR rate must be controlled in dependence on the engine operating condition and high precision is required of the control especially when high EGR rates are involved in the scope of the control.
In conventional EGR control systems, it is a usual way of operating an EGR control valve to employ a vacuum-operated actuator which is connected to the induction passage of the engine, so that the EGR control valve is operated in dependence on the magnitude of vacuum either at a venturi section of the induction passage or at a section near a throttle valve. Such vacuum is of course an indication of the volume flow rate of air in the induction passage, but there is a limitation to the precision in the control of EGR when the actuator (hence the control valve) is directly operated by, for example, the venturi section vacuum. Furthermore, it is inevitable that a considerable fluctuation occurs in the EGR rate upon variation of the engine operating condition, causing instability of the engine operation and/or increase in the emission of NOx.